In automatic and semiautomatic weapons there is normally found recoiling parts such as the bolt, carrier, etc., which reciprocate between a recoil position, such as after a cartridge has been fired, and an in-battery position, ready for the cartridge to be fired. The purpose of this reciprocating movement is to absorb recoil energy and to facilitate chambering of a fresh cartridge. The recoiling parts are urged toward the in-battery position by a helical action spring. The distance the bolt and carrier must travel dictates a fairly long spring must be used. An undesirable characteristic of these springs is that they do not compress evenly along the length of the spring at the high velocities attained by the recoiling parts of the weapon. This uneven compression induces a shock wave in the spring which is known as spring surging. This condition greatly reduces spring life and efficiency.
A simple solution to the spring surging problem would seem to be to reduce the weapon cyclic rate and hence the spring compression velocity. However, this is usually not practical or possible for several reasons. In fully automatic weapons, for example, the desired rate of fire determines the cyclic rate of the weapon and thus the velocity of the recoiling parts of the weapon. Another factor which must be considered in determining the recoil velocity is the amount of friction and other parasitic forces to be overcome. The recoiling parts usually perform several functions, for example, extracting and ejecting the fired round, searing the firing mechanism, and stripping and chambering a fresh round. The recoiling parts must have sufficient energy to accomplish this. Since velocity is proportional to energy, the velocity range is further restricted. Therefore, the cyclic rate cannot be reduced simply to overcome spring surging.